1. Field of the Invention
The present invention relates to an optical recording medium in which a recording layer optically changes to record data when a blue or blue-violet laser beam is irradiated thereon.
2. Description of the Related Art
Optical recording media such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely utilized as information recording media. Furthermore, in recent years optical recording media which utilize a blue or blue-violet laser beam of a wavelength of 405 nm (or in the range of 380 to 450 nm) as the irradiating light so that they can record more data with higher density are becoming increasingly popular.
Optical recording media are roughly categorized as a ROM (Read Only Memory)-type which can neither record nor rewrite data, an R (Recordable)-type which can record data only once, and an RW (Rewritable)-type which can rewrite data. The features that are particularly important regarding the recording layer of an R-type optical recording medium are that its optical properties change when irradiated with laser beam light, and, moreover, its quality hardly deteriorates even after being stored for a long period of time, providing superior durability. Organic dyes have been conventionally used as materials for the recording layer of an R-type optical recording medium. These conventional organic dyes have a tendency to absorb less ultraviolet light or visible light of a short wavelength such as blue or blue-violet, which are apt to promote chemical reactions, and this characteristic has contributed to the inhibition of any deterioration.
However, since conventional organic dyes do not absorb light of a short wavelength such as blue or blue-violet light very well, if a blue or blue-violet laser beam is used as the irradiation light, there is insufficient change in the optical properties of the recording layer to allow for the satisfactory recording of any data. Furthermore, it has been difficult to develop organic dyes which produce enough change in the optical properties of the recording layer in the case where a blue or blue-violet laser beam is used as the irradiation light.
On the other hand, R-type optical recording media in which inorganic materials such as oxides, and the like of, for example, Bi, Mo, Cu, Ag or Pd are used as the materials for the recording layer are known (for example, see Japanese Patent Laid-open Publication No. 2003-48375 and Japanese Patent Laid-open Publication No. Hei 10-334507). Some of these inorganic materials are capable of producing changes in the optical properties even if a blue or blue-violet laser beam is used as the irradiation light, but it was difficult to ensure that enough change in the optical properties is obtained in order to record data.
In order to overcome this difficulty, it is possible to improve the optical properties of the recording layer by forming dielectric layers, or the like on both sides of the recording layer.
However, formation of dielectric layers or the like would not only complicate the structure of an optical recording medium but also increase the number of production steps, thereby lowering productivity and increasing the cost of manufacturing the optical recording media. Furthermore, in the case where recording layers, dielectric layers or the like of inorganic materials are to be formed, an expensive vacuum deposition apparatus such as a sputtering apparatus has to be used. In order to form recording layers, dielectric layers, or the like made of different materials, a plurality of vacuum deposition apparatuses corresponding to the number of differing materials would need to be provided, thereby increasing the cost of manufacturing optical recording media even further.